That is the question Banta’s family is asking after they learned Thursday afternoon that the 22-year-old was shot and killed by Walnut Creek police early that morning.

Banta’s grandfather, Brett Grainger, he thinks his grandson may have had some sort of mental breakdown the morning he was shot, saying over and over that, “Anthony was not himself.”

“You hear many stories about many people, hearing they were ‘perfect,’ but honestly, Anthony was,” Grainger said. “This kid was upstanding. The most trouble he ever had was a speeding ticket.”

Banta was shot and killed by Walnut Creek police after they were sent to the Diablo Pointe Apartments at 1450 Creekside Drive about 3:15 a.m. Thursday. Police didn’t provide specific details about the incident, but Grainger said that the girlfriend of Banta’s roommate called 911, telling dispatchers that Banta was attacking her boyfriend. Dispatchers could hear screaming in the background, police have said.

Grainger said Banta was armed with a knife when police arrived and that multiple officers shot his grandson several times.

Police did not return calls for comment, but at a news conference Thursday, Walnut Creek police Capt. Tim Schultz said officers confronted an armed Banta at the apartment and “were forced to shoot.”

Banta, who worked as a hairdresser at Salonamour Hair Salon in Walnut Creek, was not an aggressive or threatening person, Grainger said, and before he apparently attacked his roommate, nothing about the evening had been unusual.

Banta had cooked dinner for his roommate and his girlfriend and then had gone for a walk before turning in for the night around 9 p.m., Grainger said he was told.

In a few hours, everything changed.

“He woke up, screaming and out of his mind,” Grainger said. “He didn’t recognize his roommate, and when his roommate asked him why he was choking him, he said, ‘I’m not Anthony.'”

Grainger speculates Banta possibly had a brain tumor, or some sort of mental breakdown, adding that others in Banta’s family have dealt with mental issues in the past.

Grainger also wants to know why police used such force on his grandson.

“They did not have to resort to (lethal force),” he said. “Why they couldn’t have stopped him? I have no idea.”

Grainger said Banta’s family is still reeling from the news, and Banta’s twin brother, who is in the U.S. Air Force and stationed in Germany, is flying back home.

Banta was a sweet, gentle man who told his grandfather repeatedly every time he called him that he loved him, said Grainger, who lives in Paradise, near where Banta and his brother grew up.

The grandson he knew was not the man with the knife police found Thursday morning, Grainger said.

“I had a great deal to do with his upbringing, and he was the son I always wanted and in a way, the wonderful son I got,” Grainger said. “Our family will never be the same.”

Staff writer Daniel Jimenez contributed to this report. Contact Katie Nelson at 925-945-4780 or follow her at Twitter.com/katienelson210.